Metastasis is the ultimate cause of mortality in breast cancer patients, developing in the bone more frequently than any other site. Moreover, bone metastasis occurs in approximately 80% of advanced breast cancer patients and causes considerable morbidity in the form of bone pain, pathological fractures, nerve compression, and life-threatening hypercalcemia. Sadly, bone metastasis is frequently incurable and the median survival time following diagnosis is just 2 years. Despite ongoing research efforts, the molecular and cellular mechanisms that regulate the establishment of bone metastasis and tumor-induced osteolysis remain poorly understood. Identification of factors regulating these events would be ideal targets for preventative and therapeutic interventions against this devastating disease. Using a mouse model of breast cancer metastasis, we identified the transcriptional co-activator CITED2 as a potential mediator of bone metastasis. Reducing CITED2 levels in mouse mammary tumor cells significantly reduced the establishment of bone metastasis and osteolysis in vivo. Supporting a role for CITED2 in human breast cancer, CITED2 levels in human primary breast tumors were inversely correlated with survival and were significantly elevated in metastasis, with highest levels in bone metastasis. Further, in preliminary studies, increased levels of CITED2 in human breast cancer cells significantly reduced survival time, increased incidence of hind limb paralysis, and elevated bone destruction in mice. In addition, CITED2 induced the expression of Runx2 as well as the putative Runx2 targets, OCL-activating factors IL-8, COX-2, and PTHrP, and was localized to the Runx2 promoter in human breast cancer cells, indicating that CITED2 may positively regulate transcription of Runx2, a reported mediator of bone metastasis. Based on these preliminary data, we hypothesize that CITED2 plays a key role in mediating human breast cancer metastasis to bone and pathological bone loss. We will initially investigate our hypothesis by determining the effect of increasing or decreasing CITED2 expression on the ability of human breast cancer cells to establish bone metastasis and induce bone destruction in mice following i) growth in the mammary gland, ii) administration into the vascular system, and direct administration into the bone. Next, we will examine the role of CITED2 in regulating Runx2 expression in a series of experiments identifying the co-factor(s) through which CITED2 enhances Runx2 transcription. Subsequently, we will determine whether Runx2 and its putative targets IL-8, COX-2, and PTHrP mediate the pro-metastatic effects of CITED2 on human breast cancer cells by examining the effect of increasing or decreasing expression of these factors on the ability of CITED2 to enhance in vitro migration and invasion, and in vivo colonization of bone and bone destruction. If successful, these studies will identify a novel mediator of bone metastasis and a new avenue for its prevention and treatment.